<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }
        .box {
            width: 400px;
            height: 200px;
            margin: 100px auto;
        }
        .left {
            width: 200px;
            height: 200px;
            float: left;
            background-color: red;
            padding-left: 10px;
            box-sizing: border-box;
        }
        .right {
            width: 200px;
            height: 200px;
            float: left;
            background-color: blue;
        }
    </style>
</head>
<body>
    <!--  -->
    <div class="box">
        <div class="left">border-box: 左边有10px的padding  内容(我)的宽度减少了10px</div>
        <div class="right"></div>
    </div> 
</body>
</html>